Technical Articles
Brief Introduction of Pressure Seal Valves
Posted: 2013-07-20 17:52:13 Hits: 1137
Pressure Seal Valves
Pressure seal construction is designed for valves for high pressure services, generally in excess of above 170 bar. The unique feature of the pressure seal Bonnet is that the body-Bonnet joints seals improves as internal pressure in the valve increases, compared to other constructions where the increase in internal pressure tends to create leaks in the body-Bonnet joint.
The higher the internal pressure is, the greater the sealing forces are. Easy dismantling is made possible by dropping the Bonnet assembly into the body cavity and driving out the four-segmental thrust rings by a push pin.
Relying on fairly simple design principles, pressure seal valves have proven their capability to operate increasingly demanding fossil and combined-cycle steam isolation applications, as designers continue to push boiler, HRSG, and piping system pressure/temperature envelopes. Pressure seal valves are typically available in sizes range from 2 inches to 24 inches and ASME B16.34 pressure classes from #600 to #2500, although some manufacturers can accommodate to the need for larger diameters and higher ratings for special applications.
Pressure seal valves are available in many materials such as A105 forged and Gr. WCB cast, alloy F22 forged and Gr.WC9 cast; F11 forged and Gr.WC6 cast, austenitic stainless F316 forged and Gr.CF8M cast; for over 500°C, F316H forged and suitable austenitic cast grades.
The design concept of pressure seal can trace back to the mid-1900s. Facing with constantly increasing pressures and temperatures (primarily in power applications), valve manufacturers began to try to convert the traditional bolted-Bonnet approach to sealing body/Bonnet joint. Along with a higher level of pressure boundary sealing integrity provided, many of the pressure seal valve designs weighed significantly less than their Bolted Bonnet valve counterparts.
Bolted Bonnets vs. Pressure Seals
To understand the pressure seal design concept better, it requires a comparison between bolted Bonnets and pressure seals on the basis of body-to-Bonnet sealing mechanism. Firstly, we should have a look at the typical Bolted Bonnet valves. The body flange and Bonnet flange are joined by studs and nuts, with a gasket of suitable design/material inserted between the flange faces to facilitate sealing. Studs/nuts/bolts are tightened to prescribed torques in a pattern defined by the manufacturer to affect optimal sealing. However, as system pressure increases, the potential for leakage through the body/Bonnet joint also increases.
Pressure Seal Gaskets
One of the primary components involved in sealing pressure seal valves is the gasket. Early pressure seal gaskets were manufactured from iron or soft steel. These gaskets were subsequently silver-plated to provide a tighter seal, taking advantage of softer plating material's ability. Due to the pressure applied during the valve's hydrotest, a "set" (or deformation of the gasket profile) between the Bonnet and gasket was taken. Because of the inherent Bonnet take-up bolt and pressure seal joint elasticity, the potential for the Bonnet to move and break that "set" when subjected to system pressure increases/ decreases existed, with the result of body/Bonnet joint leakage.
This problem could be effectively negated by excising the practice of "hot torquing"; the Bonnet take-up bolts after system pressure and temperature equalization, but it required owner or user or maintenance personnel to do so after plant started up. If this practice was not persisted, there comes the feasibility of leakage through the body/Bonnet joint, which could damage the pressure seal gasket, the Bonnet and/or the I.D. of the valve body, and bringing a sequence of problems and inefficiencies that the steam leakage could have on plant operations. As a result, valve designers took several steps to cope with this problem.
Secondly, a combination of live-loaded Bonnet take-up bolts (thus maintaining a constant load on the gasket, minimizing the potential for leakage) and the replacement of the iron/soft steel, silver-plated pressure seal gasket with one made of die-formed graphite are shown. The gasket design shown in Figure 3 can be installed in pressure seal valves previously and supplied with the gasket of traditional type. The advent of graphite gaskets has further enhanced reliability and performance of the pressure seal valve in most applications, even for daily start-or-stop operating cycles.
Many manufacturers still appeal to the "hot torquing," but when not conforming to it, the potential for leakage is greatly diminished. Comparatively speaking, the seating surfaces in pressure seal valves, as in many power plant valves, are bale to support higher seating loads. Seat integrity is maintained as a function of tight machining tolerances on component parts, which means providing requisite torque to open or close as a function of gears or actuation, and selection/ application of proper materials for seating surfaces.
Cobalt, nickel, and iron-based hardfacing alloys are utilized for optimal wear resistance of the wedge/disc and seat ring seating surfaces. What being most commonly used are the CoCr-A (e.g., Stellite) materials. These materials are applied with a variety of processes, including shielded metal arc, gas metal arc, gas tungsten arc, and plasma (transferred) arc. Many pressure seal globe valves are designed with integral hard-faced seats, while in general, the gate valve and check valves have hard-faced seat rings which are welded into the valve body.
Valve terminology
As long as you have ever dealt with valving, you've probably noticed that valve manufacturers are not overly creative with the terms and vernaculars used in business, such as "bolted Bonnet Valves." The body is bolted to the Bonnet to maintain system integrity. For "pressure seal valves," system pressure aids the sealing mechanism. For "stop/Check Valves," when the valve stem is in the closed position, flow is mechanically stopped, but when in the open position, the disc is free to check a reverse flow. This same principle applies to other terminology used for design, as well as valve types and their component parts.
